Locations and morphology

Sebaceous glands can usually be found in
hair-covered areas, where they are connected to hair
follicles. The glands deposit sebum on the hairs, and bring it
to the skin surface along the hair shaft. The structure consisting
of hair, hair follicle, arrector pili muscle, and sebaceous gland
is known as a pilosebaceous unit.

Sebaceous glands are also found in non-haired
areas (glabrous skin) of eyelids, penis, labia
minora, and nipples.
Here, the sebum traverses ducts which terminate in sweat pores on
the surface of the skin. The inner surface of the lip contains
sebaceous glands also, here they are common as Fordyce's
spots.

At the rim of the eyelids, meibomian
glands are a specialized form of sebaceous gland. They secrete
sebum into the tears
coating the eye, to slow
evaporation.

Sebum

Sebaceous glands secrete an oily substance called
sebum (Latin,
meaning fat or tallow) that is made of fat
(lipids) and the debris of
dead fat-producing cells. In the glands, sebum is produced within
specialized cells and
is released as these cells burst; sebaceous glands are thus
classified as holocrine glands. Sebum itself
is odorless, but its bacterial breakdown can produce
odors. Sebum is one cause of some people experiencing "oily" hair
or skin if not washed for several days. Earwax is partly
composed of sebum.

Sebum acts to protect and waterproof hair and skin, and keep them from
becoming dry, brittle, and cracked. It can also inhibit the growth
of microorganisms on skin.

Composition

The composition of sebum varies from species to
species; in humans, the lipid content is as follows:

Changes during development

The sebaceous glands of a human
fetusin utero secrete
a substance called Vernix
caseosa, a "waxy" or "cheesy" white substance coating the skin
of newborns.

The activity of the sebaceous glands increases
during puberty because
of heightened levels of androgens. In males, sebaceous
glands begin to appear predominantly on the penis during and after puberty.
This is normal, however, and not to be confused with an
STD. In females, they appear predominantly in the labia
minora.

Pathology

Sebaceous glands are involved in skin problems
such as acne and
keratosis
pilaris. The prescription drug isotretinoin significantly
reduces the amount of sebum produced by the sebaceous glands, and
is used to treat acne.

The extreme use (up to 10 times doctor prescribed
amounts) of anabolic
steroids by bodybuilders for muscle
gain and repartitioning effects tend to stimulate the sebaceous
glands which can cause acne.